This Application is a U.S. National Phase Application of PCT International Application PCT/JP00/01228.
The present invention relates to a chip positive temperature coefficient (hereinafter, PTC) thermistor comprising conductive polymers having PTC properties.
When overcurrent is applied in an electric circuit, conductive polymers with PTC properties spontaneously heat up and thermally expand to become a high resistance polymers, thereby lowering the current to a safe low-current level. As such, PTC thermistors can be used as an overcurrent protection element.
One of the conventional chip PTC thermistor configurations is disclosed in the Published Japanese Translation of PCT Publication No. H09-503097. FIG. 18(a) is a sectional view of the conventional chip PTC thermistor, and FIG. 18(b), a top view. The PTC thermistor comprises:
a resistive element 1 which is made with conductive polymer having PTC properties;
electrodes 2a and 2b, and 2c and 2d made with metal foil formed respectively on the front and back faces of the resistive element 1;
a pair of through-holes 3 having openings 3a and 3b which penetrate through the resistive element 1; and
conductive members 4a and 4b formed by plating on the internal walls of the through-holes 3 in such a manner that they electrically connect the electrodes 2a and 2d, and 2b and 2c. 
Another chip PTC thermistor which achieves soldered sections when mounted on a circuit board and allows flow soldering. As shown in FIG. 19(a), a perspective view, FIG. 19(b), a sectional view, and FIG. 19(c), exploded perspective view, the chip PTC thermistor comprises;
a conductive polymer sheet 5 having PTC properties;
electrodes 6a and 6b, and 6c and 6d made with metal foil formed respectively on the front and back faces of the conductive polymer 5; and
side face electrodes 7a and 7b formed by plating on the side faces of the conductive polymer 5 in such a manner that they electrically connect the electrodes 6a and 6d, and 6b and 6c. The conductive polymer 5 is a mixture of polymeric materials such as polyethylene and carbon black.
The conductive polymer 5 of the PTC thermistor expand spontaneously due to the heat (heat energy P=I2xc3x97R, I: current, R: PTC thermistor resistance) generated when overcurrent is applied, resistance. In the case of the chip PTC thermistor of the present invention, the electrodes 6a and 6c restrict expansion of the conductive polymer sheet 5 in the perpendicular direction, the same direction of the current passage. This prevents the rate of increase in resistance of the PTC thermistor from increasing to the capacity of the conductive polymer 5. Consequently, the range of the increase in resistance, keeps the balance of the power consumption (P=V2/R, V: applied voltage), low, thereby preventing the voltage from rising.
The chip PTC thermistor of the present invention comprises;
a conductive polymer having PTC properties;
a first main electrode disposed on and in contact with the conductive polymer;
a second main electrode disposed sandwiching the conductive polymer with the first main electrode;
a first electrode electrically connected to the first main electrode;
a second electrode electrically connected to the second main electrode; and
a means for releasing restriction against deformation comprising a cut-off section or a opening, disposed at least on one of the first and second main electrodes.
Since this construction comprises the means for releasing restriction against deformation, expansion of the conductive polymer to the perpendicular direction can be facilitated when overcurrent is applied to the chip PTC thermistor. As such, the resistivity of the conductive polymer increases, pushing up the rate of increase in resistance. Therefore, performance of the chip PTC thermistor in increasing resistance improves, thereby enhancing withstand voltage.
As the need arises, odd or even-numbered inner electrodes can be disposed in between the first and second main electrodes.
In the case of the chip PTC thermistor of the present invention, it is desirable to dispose the means for releasing restriction against deformation in the vicinity of the joints between the main electrodes and the first and second electrodes, in such a manner that each of the adjacent means being disposed symmetrically to the center of the space between the first and second electrodes. This construction allows the conductive polymer to expand more easily, thus further facilitating increases in its resistance and withstand voltage.
The means for releasing restriction against deformation formed on the main electrode should be preferably disposed rotationally symmetrically on a face parallel to the main electrode. This construction averages the distortion of the PTC thermistor caused by the expansion of the conductive polymer, thereby enhancing reliability.
The means for releasing restriction against deformation should preferably be made with an opening or a cut-off section. The opening or a cut-off section helps the conductive polymer to expand, thus further facilitating increases in resistance.
According to the chip PTC thermistor of the present invention, it is preferable to provide a first sub-electrode on a same plane of the first main electrode in such a manner that the first sub-electrode is electrically separated from the first main electrode and electrically connected to the second electrode.
Preferably, the first electrode is a first side electrode disposed on one of the side faces of the conductive polymer while the second electrode is a second side electrode disposed on the other side face of the conductive polymer.
The first and second electrodes can be respectively first and second internal through electrodes penetrating through the conductive polymer.
The first electrode can also comprise the first side electrode disposed on one of the side faces of the conductive polymer and the first internal through electrode penetrating through the conductive polymer while the second electrode comprises the second side electrode disposed on the other side face of the conductive polymer and the second internal through electrode penetrating through the conductive polymer as well.